Heretofore, a mobile crane to/from which a luffing jib can be attached and removed is known. The luffing jib extends the workable range of the mobile crane by being attached to the tip of a boom of the mobile crane. Japanese Patent No. 3257508 describes a procedure of attaching the luffing jib to the mobile crane.
According to Japanese Patent No. 3257508, the luffing jib placed on the ground is first connected to the tip of a boom. Next, a jib derrick winch is hoisted up, so that a front post and a rear post are raised to reach predetermined rotation angles. Next, a boom derrick winch is hoisted up, so that a boom is raised to reach a predetermined derrick angle. Then, the jib derrick winch is further hoisted up, so that the luffing jib is raised to reach a predetermined tilt angle.
As described also in Japanese Patent No. 3257508, when the operation is not carried out according to the above-described procedure, the constituent parts of the mobile crane may be damaged. For example, when a boom 22 is raised in a state where a first mast 39 and a second mast 40 are not sufficiently raised as illustrated in FIG. 11 and FIG. 12, the second mast 40 and a rope 44 are substantially in parallel to each other and the second mast 40 is rotatable with respect to a support 38.
Then, when the boom 22 is further raised from the state illustrated in FIG. 12, the first mast 39 and the second mast 40 may fall down to the luffing jib 30 side. In this case, when the tilt angle (angle formed by the boom 22 and the luffing jib 30) of the luffing jib 30 is large, the luffing jib 30 (a base jib 31 in more detail) may be damaged by the fallen-down second mast 40.